Gear milling cutters of the above-mentioned type are foremost used for the roughing of metallic workpieces with the purpose of providing gear paths in miscellaneous machine elements, such as gear wheels, toothed plates, racks, etc. In industrial production, such rough milling is usually carried out by the workpiece being clamped in a stepwise movable holder, whereupon the milling cutter is fed rectilinearly (e.g. vertically) under rotation through a part of the workpiece while milling out an individual gash or tooth gap. This procedure is repeated by the holder being moved stepwise up to new positions in relation to the milling cutter, which mills out further gaps until a desired number of teeth or cogs have been created. The result of this initial rough milling becomes teeth or “tooth blanks” having a shape that reasonably adheres to the nominally prescribed, exact shape of the tooth, but that has not the final dimensional accuracy. Therefore, it is necessary to finish the teeth in one or more finishing operations, e.g. fine milling, grinding, hobbing, or the like, in order to finally achieve the desired dimensional accuracy. In this connection, it should be pointed out that the requirements of dimensional accuracy within the technical field in question often are about thousandths, rather than hundredths, of a millimeter. Therefore, in the concluding finishing process, it is about to remove tenths or hundredths of a millimeter to achieve prescribed tolerances in the gear paths.
In the gear milling cutter—as well as in other chip removing tools making use of replaceable, hard milling inserts—a varying number of sources of error are present that may disturb or entirely jeopardize the machining precision of the tool. For instance, the outcome in the production of the milling inserts may vary so far that the milling inserts at times swell and at times shrink in relation to their nominal, calculated dimensions (the errors are within the range of ±0.5%). Neither can it with absolute certainty be guaranteed that the seats in the basic body, in which the milling inserts are mounted, obtain their exact desired, solid geometrical positions in the same. Other sources of error that may affect the flank surfaces of the rough-milled teeth are, on one hand, the state (age) of the driving machine tool, and on the other hand the risk of emergence of vibrations.
A disadvantage of previously known gear milling cutters (see e.g., JP 2002144129 A) is the difficulty to master the inevitable surface imperfections that regularly arise in the flank surfaces of the teeth in connection with the initial rough milling. Such surface imperfections arise above all in the zones where the sweep areas of the milling inserts overlap each other, and may alternately manifest themselves in the form of convexities (e.g. ridges, bulges and the like) or concavities (e.g. grooves, pits, level differences) in the milled surface. With known gear milling cutters, the character of these surface imperfections cannot be predicted; something which later may make the concluding finishing operation more difficult. In practice, convexities are relatively simple to eliminate by, for instance, being ground down in relation to the surrounding surface, without the tolerances of the teeth being jeopardized. However, concavities are considerably more delicate because the surrounding surface layer has to be ground down or be removed until a smooth, dimensionally accurate flank surface is obtained. If the rough-milled flank surfaces—as most often is the case—include concavities as well as convexities that alternate with each other helter-skelter in unpredictable patterns, the concluding finishing operation becomes particularly awkward. In other words, there is a considerable risk of the prescribed tolerances not being achievable in connection with the concluding finishing operation; something that in the worst case may cause rejection of the workpiece. Such rejections are highly undesired considering that the finished product at times has a very high economical value.
The present invention aims at obviating the above-mentioned disadvantages of previously known gear milling cutters and at providing a gear milling cutter by which the surface imperfections in the rough-milled teeth can be predicted and thereby overcome.
An object of the invention to provide a gear milling cutter that on rough-milled teeth generates surfaces, above all flanks, which have a predictable shape and the inevitable surface imperfections of which are limited to convexities, contrary to concavities.
It should already here be emphasized that the invention in no way claims to be able to eliminate the concluding finishing operation. On the contrary, the invention is based on the understanding that each roughing gear milling cutter inevitably is impaired by sources of error, which make impossible absolute machining perfection, and that a concluding finishing operation always is required. Thus, the primary purpose of the invention is to facilitate the realization of the final machining while ensuring that desired tolerances of gear paths in expensive workpieces are maintained.